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PiezoTone: Piezoelectric Microphone
MEMS Project
Gabriel Fernandez1
, Colburn Schacht2
, Steve Monroy3
, Craig Thompson4
EEE 4463 – MEMS Devices and Applications
Abstract— This paper will go over the marketing,
design process, analytics, and manufacturing of the
MEMS microphone to be produced by our company,
PiezoTone. Along with our initial findings, additional
information was revised and included. As numbered
above, Gabriel Fernandez will write the first portion
of this paper covering marketing. The second portion
of this paper will be written by Colburn Schacht and
will cover the design process of the microphone.
Steve Monroy will cover the third portion of the
paper which will cover analyzing the design of the
microphone. The fourth portion of this paper will be
covered by Craig Thompson who will write about the
theoretical fabrication process of the design. Finally,
Gabriel Fernandez will sum the ideas into a
conclusion.
Index Terms:
GPRO – GoPro, Inc.
SNE- Sony Corporation
Mo – Conductor, Molybdenum
ZnO – Piezoelectric material, Zinc Oxide
MEMS – Micro Electric Machine System
SF6 – Sulfur Hexafluoride
DRIE – Deep Reactive Ion Etching
I. Marketing
II. Design
III. Analysis
IV. Manufacturing
V. Conclusion
I. MARKET
A. Overview of MEMS Microphone Market
[1] Global MEMS microphone shipments expanded
to 2.6 billion units in 2013, up 37% from 1.9 billion in
2012. By 2017, shipments will reach 5.4 billion units.
[2] Knowles accounts for 59% of the market's total
revenue among packaged MEMS microphone suppliers.
Currently the majority of MEMS microphones used,
are capacitive based microphones. This is the area where
Knowles has the edge. [3] Having implemented a
cantilever style “diaphragm”, they managed to get rid of
the problems associated with “diaphragms”.
Just like with the rest of MEMS applications, there
are still new markets that MEMS microphones can tap
into. This will be the premise of our market strategy.
Instead of competing with other manufacturers in more
crowded areas our main focus was to expand the use of
MEMS microphones to new areas.
B. PiezoTone’s Target Market
After multiple ideas and extensive considerations
PiezoTone decided to target markets in need of
ruggedized microphones. Unlike Knowles and other
companies, PiezoTone decided to stay clear of the high-
fidelity market by developing a product, which is both
inexpensive and able to handle extreme and
unpredictable conditions.
This “rugged” aspect is possible thanks to our focus
on Piezoelectrics. By developing a strictly piezoelectric
microphone we were able to get rid of most mechanical
structures and in turn have a device that uses virtually no
power. One of the multiple areas where these types of
microphones might be useful is the action camera
market.
This is a relatively new market and one of great
importance to major companies like GPRO and SNE. [4]
The former went public on June 25, 2014 with an

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expected range of $21 to $24 a share, selling 17.8
million shares to initial investors at $24 per share
(totaling $427.2M). [5] At the IPO price the company
was valued at $2.95 billion. Currently GPRO has a
market cap of $8.35B and one of the most promising
expected growths.
SNE can be considered a newcomer to the market,
but with their advanced lens technology and big coffers
they’ve quickly established themselves in this market.
Much to PiezoTone’s advantage, both of these
company’s action camera offerings suffer from poor
battery life and sound. These are the two main selling
points of our device and constitute the focus of our
microphone.
C. Costs
The standard has been that piezoelectric devices cost
more than capacitive. Although this is still the case, the
constant improvement of wafer and die sizes will soon
make piezoelectric materials cheaper.
In order to benefit our potential buyers with a
ruggedized microphone that’s still effective, easy to
manufacture and low cost, we chose materials that are
among the cheapest to work with. Zinc (Zn) alone is
$1.08 per pound. Compared to Knowles’ Piezo Ceramic
line of microphones (our closest competitor); which
retail for at least $41.327, our device would be a more
viable option for companies interested in ruggedness
over fidelity.
By providing one of the cheapest alternatives in the
market, and tapping into new markets altogether,
PiezoTone should be able to quickly establish itself as
one of the main manufacturers of piezoelectric
microphones.
II. DESIGN
A. Preliminary Process
Fig. 1 – Diagrams of both a directly actuated (A) and a
diaphragm (B) piezoelectric microphone. These were the main
forms of which we based our first design off of.
Our search for a design began with research of the
current designs for using piezoelectric sensing to create a
MEMS microphone. The two main designs we found
were of cantilever structure and can be seen in Fig. 1.
The first design we found used the force directly from the
sound waves to produce the electric signals that
correspond to the frequency of the incoming waves. The
second design functioned in a similar way, however used
a shorter fixed-free cantilever with a diaphragm
separating the incoming sound waves from the sensor
that focused the force of the sound wave onto one point
on the end of the cantilever. Using these two designs as
reference, we came up with the first of our two
microphone designs, the second being our final design.
Fig. 2 – First Microphone design featuring the conical entrance
for the incoming sound waves
As pictured above, our first design is that of a fixed-
free cantilever made of silicon with our piezoelectric
material located on top of the fixed end of the beam. On
the other end, there is located above it a conical entrance
to focus the incoming sound waves towards a singular
point on the beam. This focus would, in theory, allow the
microphone to have a higher sensitivity and lower noise
floor, however after some testing we realized the focus
did just the opposite. The conical shape would cause the
sound waves to rebound too heavily into one another and
strike the cantilever at a frequency differing from the
original message. Along with that, the noise floor was
raised above a lone beam with no cone because the
rebounding effect of the sound waves caused unnecessary
vibrations and resonance in the system.

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Fig. 3 – University of Michigan Microphone we used for
reference
After we realized the error in our initial design, we
decided to stick with a basic cantilever design, stripping
the original design of the cone and determining an
effective orientation to place our cantilevers in. [7] above
is a picture of an orientation that proved effective that
was tested by researchers at the University of Michigan.
They used large quantities of thin cantilever beams
placed side by side one another to create a higher
resolution microphone. The design was enticing to us,
however the big problem we saw with their design was a
fragility that would not benefit a microphone for our
intended market. So using their microphone as reference,
we designed our second iteration of the PiezoTone
Microphone.
B. Final Design
Fig. 4 – Final design of the PiezoTone Microphone featuring
the side by side orientation of the four wide cantilevers
In the above picture is our design for our PiezoTone
Microphone. The design features four wide, fixed-free
cantilevers placed across from one another in pairs. The
four cantilevers are identical to one another, each being
made entirely of silicon, with the piezoelectric material
placed at the top of the fixed end of each beam. The
space between each of the cantilevers is miniscule
relative to the cantilevers themselves to allow the beams
to read the incoming sound waves as best as possible.
The design changes of lowering the cantilever count
and increasing the width, as well as making the material
composition of the cantilevers silicon, instead
piezoelectric material, as the University of Michigan
example had, gives us a microphone more applicable to
the market we are targeting. The silicon cantilevers
would be a more durable, predictable alternative to the
piezoelectric cantilevers because of the simplicity of
design. With the four wide cantilever design we also
increase the acoustic resistance of our microphone as a
whole, meaning the microphone encounters more of the
incoming sound waves, increasing the accuracy of the dB
readings, while also eliminating an issue that the
researchers at the University of Michigan feared, where if
the cantilevers were made too wide, they would twist and
oscillate in a torsional manner, throwing off readings.
Another factor that the silicon beams have over the
piezoelectric beams is that of cost, where the cost of
producing the solid silicon cantilevers, as compared to
fabricating the beams of piezoelectric material, would be
significantly less. The amount of piezoelectric material
necessary would also serve to lower the cost of
production even further.
Durability and cost of creation were the focuses under
which we designed the PiezoTone microphone. Other
factors, such as quality of sound, were important as well
however less so than the aforementioned because of the
circumstances in which the microphone would be placed.
These advantages would allow us to slip into the action
camera market quickly and make a large impact due to
the advantages over the current mics implemented in this
field.
III. ANALYSIS
Since our design will be using four fixed-free
cantilever beams we were able to use equations found in
the textbook. When going through the equations we
decided it would be best that the value for force used
would be that of normal conversation values. These
values include the pressure ratio (P/Pr), intensity ratio
(I/Ir), reference level (Ir) and the sound level (dB). These
values in the same order are 3.16x103
, 107
,10-12
, 70.
In order to obtain a value for the incoming force we
decided to use normal conversation values. For normal
conversation the human voice produces around 60-80
dB’s so for our calculations we will be using 70 dB’s. To
convert this to Newton’s we must first convert to

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Pascal’s. Since we know that 94 dB’s = 1 Pa we found
that 70 dB’s = 0.063 Pa. Now since 1 Pascal is equal to 1
Newton we are able to use a force of 0.063 Newton’s in
our calculations.
The following equations were found in the textbook
and are for a single fixed-free cantilever beam. To do the
analysis for the entire design more analysis will be
needed. One must realize that since there will be four
beams instead of only one that the incoming force will
be spread out between the four beams making the stress
of each individual beam smaller. This is one of the
reasons we have decided to use this type of design. We
feel that it will help us achieve our goal of a sturdy and
efficient piezoelectric microphone.
To solve for the max stress of a beam we used the
max stress equation for a fixed-free cantilever beam
found in the textbook,
Ơmax = (Fltbeam)/(2Ibeam) (1)
To solve this equation we need the incoming force,
which will be the normal conversation values converted
to an applicable force, the length of where the force is
being applied on the beam, the thickness of the beam,
the permittivity of the material and the moment of
inertia. The moment of inertia is calculated using the
following equation also found in the textbook,
Ibeam = (wt3
beam)/12 (2)
To use this equation we will need the length and
thickness of the beam itself.
The overall output voltage can be found two
different ways;
V = (D3tpiezo)/ Ɛ (3)
V = (Fltbeamtpiezo)/(2ƐIbeam) (4)
where D3 = d31Ơ1. Even though there are two different
ways to solve for the output voltage both should lead to
the same result. D3 = d31Ơ1 uses the piezoelectric
coefficient for ZnO along with the stress to solve for the
electric strain. Once the electric stain is found the
thickness of the piezoelectric is used along with the
permittivity of ZnO for find the output voltage. Equation
(4) uses the incoming force, length of the beam,
thickness of the beam, thickness of the piezoelectric,
permittivity of ZnO, and the moment of inertia.
The width of both the beam and the piezoelectric
should be the same so that the piezoelectric can be able
to maximize its sensing ability across of the width of the
material. The thickness of the piezo was chosen so that it
did not alter the neutral zone of the material thus
changing its properties.
For our design we used the following values to
maximize our design; wbeam = 200µm, tbeam = .5µm, lbeam
= 500µm, wpiezo = 200µm, tpiezo = .1µm, lpiezo = 50µm and
Ɛ = 8.5. Using these dimensional values along with the
values of the material we came up with what we believe
to be a successful design. Using the stress equation
found in the textbook we were able to come up with a
stress of 3.79 TPa. Using the output voltage equation
also found in the textbook we came up with a value of
44.6 kV. Both of these values found solidified that our
design could be successful under normal conversation
circumstances.
IV. MANUFACTURING
Starting the fabrication section, this paper will go
through why the materials chosen were chosen and how
the designed MEMS microphone device will be
manufactured. The descriptions given will be detailed
with the point of view of looking at a cross section of a
single device as opposed to an entire wafer.
A. Materials
[5] We chose molybdenum because we knew we
were able to sputter it onto the wafer and were able to
keep a thin film for better efficiency in recourse and real
estate. There are also a number of ways to possibly
reduce molybdenum’s electrical resistivity for more
direct piezoelectric voltage output.
[6] Zinc Oxide was chose mainly due to its high
piezoelectric coefficient, but also due to the fact that it
can be sputtered and we know how to work with
materials that can be sputtered.
[7] Sulfur hexafluoride was chosen for the fact that
we knew it to be efficient in eliminating the silicon
dioxide layer.
B. Fabrication Process
Above is a hand-drawn representation of the
microfabrication process we will employ. The
piezoelectric material has been placed between the

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conductive materials and this package placed on top of
the cantilever. We chose for the package to be on top of
the cantilever as opposed to embedded within the
cantilever for ease of manufacturing the MEMS
microphone device and for ease of manufacturing the
casing that will enclose the device. The casing that
encloses the device will need to connect to the
conductive electrodes surrounding the piezoelectric
material.
The microphone will start as any MEMS device
would as a Silicon wafer. A layer of photolithographic
material will be placed so as to pattern a small area that
will stand as the base of the cantilever. Boron will be
implanted on wafer in the exposed portions of the
silicon. After the photolithographic layer has been
washed away, the wafer will be flipped over to expose
the bottom side of the wafer. Similar to before and
reflecting the top layer, the wafer will be coated with a
photolithographic layer with exposed patterns for the
boron doping. Once that photolithographic layer has
been washed away, the wafer will be flipped back to its
“upright” position and proceed to be oxygen baked so as
to grow a layer of silicon dioxide.
Once the silicon dioxide layer is grown, another step
of patterning with photolithographic material will be
placed upon the wafer. This time it is to expose a small
area where chemically assisted etching will take place.
This will be at the future planned free end of the
cantilever for when we eventually deep reactive ion etch
the silicon out from beneath the cantilever. So now
sulfur hexafluoride will be used as the assistive chemical
to ion etch out the tip of silicon dioxide that will become
the cantilever.
The photolithographic layer will be washed away
and patterned with another layer of photolithographic
material. This time to pattern where the molybdenum
will be placed. It will be positioned where one edge of
the molybdenum will be aligned with the edge of the
boron-doped silicon underneath the silicon dioxide layer.
The other edge of the molybdenum will be facing
towards the edge of the cantilever. This pattern was
chosen since the positioning has it on top of where the
cantilever still bends and being so close to the base of
the cantilever, the most stress will occur and thus the
most piezoelectric effects will be produced.
The photolithographic layer will be washed away
and another photolithographic layer will be placed
exactly where the last one was, but perhaps a bit thicker.
The zinc oxide will then be placed on top of the
molybdenum and the photolithographic layer washed
away. Once again, a thicker photolithographic layer will
be placed exactly where the last layer was. This time, the
top layer of the molybdenum is placed and the
photolithographic layer is washed away.
The final photolithographic layer is placed similar to
the second layer at the beginning of the manufacturing
process so as to expose the silicon laying at the tip of the
silicon dioxide cantilever. The wafer will then undergo
deep reactive ion etching so as to remove the underlying
silicon beneath the cantilever. Once the debris is washed
out along with the remains of the photolithographic
layer, the dye is ready to be cut into the many chips. The
chips’ casing will be fitted to measure the piezoelectric
effects of the zinc oxide through the electrodes of
molybdenum.
V. CONCLUSION
After a complete breakdown of our device and
company, it’s safe to assume that PiezoTone has taken a
unique approach to cater to a specific market. At the
same time, we managed to keep costs down, my utilizing
cheaper materials than other manufacturers.
By providing a detailed analysis of our design process
we are able to convey the hard work that was put into
making our final device. Having gone through various
designs we were sure to choose the right design to cater
to our future customers. Not only did we exceed our
expectations, but we designed a piezoelectric
microphone we can be proud of.
We took this even further by introducing a highly
detailed manufacturing process. Considered by some as
one of the most critical aspects of microelectronics,
implementing an effective fabrication process was
critical to our company. By detailing everything now, we
should be able to cut down on our manufacturing time
and quickly address any problems that may arise later
on.
Hopefully we at PiezoTone have convinced you that
our device has a future in this industry. Whether it’s in
the action camera market, or NASA’s next Rover, we
are sure that our device will stand the test of time.
VI. REFERENCES
[1] M. Boustany, J. Bouchaud, “MEMS Microphones
Report”, IHS Technology, 2014
[2] “In Complex Supply Chain, Knowles and Infineon are
Top Suppliers of Packaged MEMS Microphones and
Bare Microphone Dies”, IHS Technology, 2014
[3] Krantz, Matt. "GoPro IPO prices at $24, set to trade
Thursday". USA Today Money. Feb 25, 2014.

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[4] Leslie Picker. "GoPro Raises $427 Million, Pricing
IPO at Top of Marketed Range". Bloomberg. June, 26
2014.
[5] A.B. Kashyout, H.M.A. Soliman, H.A. Gabal, P.A.
Ibrahim, M. Fathy. , “Preparation and characterization of
DC sputtered molybdenum thin films,” Alexandria
University, Alexandrai, Egypt. 2010
[6] C. Liu, Foundation of MEMS. Northwestern
University, 2012.
[7] R. J. Littrell, “High Performance Piezoelectric
MEMS Microphones,” University of Michigan, MI,
2010.